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Say I have a huge text file (>2GB) and I just want to cat the lines X to Y (e.g. 57890000 to 57890010).

From what I understand I can do this by piping head into tail or viceversa, i.e.

head -A /path/to/file | tail -B

or alternatively

tail -C /path/to/file | head -D

where A,B,C and D can be computed from the number of lines in the file, X and Y.

But there are two problems with this approach:

  1. You have to compute A,B,C and D.
  2. The commands could pipe to each other many more lines than I am interested in reading (e.g. if I am reading just a few lines in the middle of a huge file)

Is there a way to have the shell just work with and output the lines I want? (while providing only X and Y)?

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1  
FYI, actual speed test comparison of 6 methods added to my answer. –  Kevin Sep 8 '12 at 2:41
    

6 Answers 6

up vote 37 down vote accepted

I suggest the sed solution, but for the sake of completeness,

awk 'NR >= 57890000 && NR <= 57890010'

To cut out after the last line:

awk 'NR < 57890000 { next } { print } NR == 57890010 { exit }

Speed test:

  • 100,000,000-line file generated by seq 100000000 > test.in
  • Reading lines 50,000,000-50,000,010
  • Tests in no particular order
  • real time as reported by bash's builtin time
 4.373  4.418  4.395    tail -n+50000000 test.in | head -n10
 5.210  5.179  6.181    sed -n '50000000,50000010p;57890010q' test.in
 5.525  5.475  5.488    head -n50000010 test.in | tail -n10
 8.497  8.352  8.438    sed -n '50000000,50000010p' test.in
22.826 23.154 23.195    tail -n50000001 test.in | head -n10
25.694 25.908 27.638    ed -s test.in <<<"50000000,50000010p"
31.348 28.140 30.574    awk 'NR<57890000{next}1;NR==57890010{exit}' test.in
51.359 50.919 51.127    awk 'NR >= 57890000 && NR <= 57890010' test.in

These are by no means precise benchmarks, but the difference is clear and repeatable enough* to give a good sense of the relative speed of each of these commands.

*: Except between the first two, sed -n p;q and head|tail, which seem to be essentially the same.

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4  
Out of curiosity: how have you flushed the disk cache between tests? –  Paweł Rumian Sep 8 '12 at 8:08
    
What about tail -n +50000000 test.in | head -n10, which unlike tail -n-50000000 test.in | head -n10 would give the correct result? –  Gilles Sep 8 '12 at 10:55
1  
Ok, I went and did some benchmarks. tail|head is way faster than sed, the difference is a lot more than I expected. –  Gilles Sep 8 '12 at 11:30
3  
@Gilles you're right, my bad. tail+|head is faster by 10-15% than sed, I've added that benchmark. –  Kevin Sep 8 '12 at 13:50
    
@gorkypl I didn't, I figure it should be cached for all of them, certainly by the third iteration. –  Kevin Sep 8 '12 at 13:52

If we know the range to select, from the first line: lStart to the last line: lEnd we could calculate:

lCount="$((lEnd-lStart+1))"

If we know the total amount of lines: lAll we also could calculate the distance to the end of the file:

toEnd="$((lAll-lStart+1))"

Then we will know both:

"how far from the start"            ($lStart) and
"how far from the end of the file"  ($toEnd).

Choosing the smallest of any of those: tailnumber as this:

tailnumber="$toEnd"; (( toEnd > lStart )) && tailnumber="+$linestart"

Allows us to use the consistently fastest executing command:

tail -n"${tailnumber}" ${thefile} | head -n${lCount}

Please note the additional plus ("+") sign when $linestart is selected.

The only caveat is that we need the total count of lines, and that may take some additional time to find.
As is usual with:

linesall="$(wc -l < "$thefile" )"

Some times measured are:

lStart |500| lEnd |500| lCount |11|
real   user   sys    frac
0.002  0.000  0.000  0.00  | command == tail -n"+500" test.in | head -n1
0.002  0.000  0.000  0.00  | command == tail -n+500 test.in | head -n1
3.230  2.520  0.700  99.68 | command == tail -n99999501 test.in | head -n1
0.001  0.000  0.000  0.00  | command == head -n500 test.in | tail -n1
0.001  0.000  0.000  0.00  | command == sed -n -e "500,500p;500q" test.in
0.002  0.000  0.000  0.00  | command == awk 'NR<'500'{next}1;NR=='500'{exit}' test.in


lStart |50000000| lEnd |50000010| lCount |11|
real   user   sys    frac
0.977  0.644  0.328  99.50 | command == tail -n"+50000000" test.in | head -n11
1.069  0.756  0.308  99.58 | command == tail -n+50000000 test.in | head -n11
1.823  1.512  0.308  99.85 | command == tail -n50000001 test.in | head -n11
1.950  2.396  1.284  188.77| command == head -n50000010 test.in | tail -n11
5.477  5.116  0.348  99.76 | command == sed -n -e "50000000,50000010p;50000010q" test.in
10.124  9.669  0.448  99.92| command == awk 'NR<'50000000'{next}1;NR=='50000010'{exit}' test.in


lStart |99999000| lEnd |99999010| lCount |11|
real   user   sys    frac
0.001  0.000  0.000  0.00  | command == tail -n"1001" test.in | head -n11
1.960  1.292  0.660  99.61 | command == tail -n+99999000 test.in | head -n11
0.001  0.000  0.000  0.00  | command == tail -n1001 test.in | head -n11
4.043  4.704  2.704  183.25| command == head -n99999010 test.in | tail -n11
10.346  9.641  0.692  99.88| command == sed -n -e "99999000,99999010p;99999010q" test.in
21.653  20.873  0.744  99.83 | command == awk 'NR<'99999000'{next}1;NR=='99999010'{exit}' test.in

Note that times change drastically if the selected lines are near the start or near the end. A command which appear to work nicely at one side of the file, may be extremely slow at the other side of the file.

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Comments are not for extended discussion; this conversation has been moved to chat. –  terdon Jul 17 at 22:46
    
@BinaryZebra - way better. –  mikeserv Jul 19 at 12:50

I do this often enough and so wrote this script. I don't need to find the line numbers, the script does it all.

#!/bin/bash

# $1: start time
# $2: end time
# $3: log file to read
# $4: output file

# i.e. log_slice.sh 18:33 19:40 /var/log/my.log /var/log/myslice.log

if [[ $# != 4 ]] ; then 
echo 'usage: log_slice.sh <start time> <end time> <log file> <output file>'
echo
exit;
fi

if [ ! -f $3 ] ; then
echo "'$3' doesn't seem to exit."
echo 'exiting.'
exit;
fi

sline=$(grep -n " ${1}" $3|head -1|cut -d: -f1)  #what line number is first occurrance of start time
eline=$(grep -n " ${2}" $3|head -1|cut -d: -f1)  #what line number is first occurrance of end time

linediff="$((eline-sline))"

tail -n+${sline} $3|head -n$linediff > $4
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You're answering a question that wasn't asked. Your answer is 10% tail|head, which has been discussed extensively in the question and the other answers, and 90% determining the line numbers where specified strings/patterns appear, which wasn't part of the question. P.S. you should always quote your shell parameters and variables; e.g., "$3" and "$4". –  G-Man Oct 8 '14 at 22:51

The most orthodox way (but not the fastest, as noted by Gilles above) would be to use sed.

In your case:

X=57890000
Y=57890010
sed -n -e "$X,$Y p" -e "$Y q" filename

The -n option implies that only the relevant lines are printed to stdout.

The p at the end of finishing line number means to print lines in given range. The q in second part of the script saves some time by skipping the remainder of the file.

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I expected sed and tail | head to be about on par, but it turns out that tail | head is significantly faster (see my answer). –  Gilles Sep 8 '12 at 11:31

The head | tail approach is one of the best and most "idiomatic" ways to do this:

X=57890000
Y=57890010
< infile.txt head -n "$Y" | tail -n +"$X"

As pointed out by Gilles in the comments, a faster way is

< infile.txt tail -n +"$X" | head -n "$((Y - X))"

The reason this is faster is the first X - 1 lines don't need to go through the pipe compared to the head | tail approach.

Your question as phrased is a bit misleading and probably explains some of your unfounded misgivings towards this approach.

  • You say you have to calculate A, B, C, D but as you can see, the line count of the file is not needed and at most 1 calculation is necessary, which the shell can do for you anyways.

  • You worry that piping will read more lines than necessary. In fact this is not true: tail | head is about as efficient as you can get in terms of file I/O. First, consider the minimum amount of work necessary: to find the X'th line in a file, the only general way to do it is to read every byte and stop when you count X newline symbols as there is no way to divine the file offset of the X'th line. Once you reach the *X*th line, you have to read all the lines in order to print them, stopping at the Y'th line. Thus no approach can get away with reading less than Y lines. Now, head -n $Y reads no more than Y lines (rounded to the nearest buffer unit, but buffers if used correctly improve performance, so no need to worry about that overhead). In addition, tail will not read any more than head, so thus we have shown that head | tail reads the fewest number of lines possible (again, plus some negligible buffering that we are ignoring). The only efficiency advantage of a single tool approach that does not use pipes is fewer processes (and thus less overhead).

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If you want lines X to Y inclusive (starting the numbering at 1), use

tail -n +$X /path/to/file | head -n $((Y-X+1))

tail will read and discard the first X-1 lines (there's no way around that), then read and print the following lines. head will read and print the requested number of lines, then exit. When head exits, tail receives a SIGPIPE signal and dies, so it won't have read more than a buffer size's worth (typically a few kilobytes) of lines from the input file.

Alternatively, as gorkypl suggested, use sed:

sed -n -e "$X,$Y p" -e "$Y q" /path/to/file

The sed solution is significantly slower though (at least for GNU utilities and Busybox utilities; sed might be more competitive if you extract a large part of the file on an OS where piping is slow and sed is fast). Here are quick benchmarks under Linux; the data was generated by seq 100000000 >/tmp/a, the environment is Linux/amd64, /tmp is tmpfs and the machine is otherwise idle and not swapping.

real  user  sys    command
 0.47  0.32  0.12  </tmp/a tail -n +50000001 | head -n 10 #GNU
 0.86  0.64  0.21  </tmp/a tail -n +50000001 | head -n 10 #BusyBox
 3.57  3.41  0.14  sed -n -e '50000000,50000010 p' -e '50000010q' /tmp/a #GNU
11.91 11.68  0.14  sed -n -e '50000000,50000010 p' -e '50000010q' /tmp/a #BusyBox
 1.04  0.60  0.46  </tmp/a tail -n +50000001 | head -n 40000001 >/dev/null #GNU
 7.12  6.58  0.55  </tmp/a tail -n +50000001 | head -n 40000001 >/dev/null #BusyBox
 9.95  9.54  0.28  sed -n -e '50000000,90000000 p' -e '90000000q' /tmp/a >/dev/null #GNU
23.76 23.13  0.31  sed -n -e '50000000,90000000 p' -e '90000000q' /tmp/a >/dev/null #BusyBox

If you know the byte range you want to work with, you can extract it faster by skipping directly to the start position. But for lines, you have to read from the beginning and count newlines. To extract blocks from x inclusive to y exclusive starting at 0, with a block size of b:

dd bs=$b seek=$x count=$((y-x)) </path/to/file
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Are you sure that there is no caching inbetween? The differences between tail|head and sed seem too big to me. –  Paweł Rumian Sep 8 '12 at 12:03
    
@gorkypl I did several measures and the times were comparable. As I wrote, this is all happening in RAM (everything is in the cache). –  Gilles Sep 8 '12 at 12:06
    
@Gilles tail will read and discard the first X-1 line seems to be avoided when the number of lines is given from the end, In such case, tail seems to read backwards from the end according to the executing times. Please read: http://unix.stackexchange.com/a/216614/79743. –  BinaryZebra Jul 17 at 4:52
    
@BinaryZebra Yes, if the input is a regular file, some implementations of tail (including GNU tail) have heuristics to read from the end. That improves the tail | head solution compared to other methods. –  Gilles Jul 17 at 7:08

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